If you want to be certain it is FUEL PROOF, use Lusterkote or Ultracote (preffered ) clear OR an automotive clear coat. Definitely test it on a test panel first to be sure of compatibility with the Krylon. I have used Ultracote clear over Krylon before with no problems.

For Fuel Resistant (up to 15% ) I have used (and still do use ) MinWax Polycrylic (water base ).

What I have found with this is to use 2 coats and let it dry for A MINIMUM of a week before exposing it to fuel. As long as you clean the plane at the end of the day you will be OK, but if you let the residue sit more than 5 or 6 hours the clear will start to soften. I also receive the impression it becomes more fuel resistant with age. I accidently missed an area on a plane and did not find it until the next day - the finish had not softened. This plane was about 20 months old at the time.

The big advantage to the water base (besides being cheap ) is that it does not yellow with age.